Support FHS 3.0

[fossfreedom]

http://refspecs.linuxfoundation.org/FHS_3.0/fhs/ch04s07.html

In summary - where we have python or vala executables in /usr/lib/budgie-desktop/appletname this is no longer "valid" in the new FHS-3.0 spec.

Meson and code will need to be updated to use the /usr/libexec/foldername concept as described in the new spec.

At the moment debian is throwing pedantic lintian.

This is a longer term issue since its going to take quite a while for all packages from everywhere to start using the new specification

[fossfreedom]

Note - this issue is easily resolved once #310 and #309 are solved so that we can pass a build parameter "USE_LIBEXEC=true" in meson.options

if true then in the top level meson.build set the LIBEXEC variable in config.h which can then be used in code

[fossfreedom]

P: budgie-dropby-applet: executable-in-usr-lib usr/lib/budgie-desktop/plugins/budgie-dropby/checkonwin
P: budgie-dropby-applet: executable-in-usr-lib usr/lib/budgie-desktop/plugins/budgie-dropby/copy_flash
P: budgie-dropby-applet: executable-in-usr-lib usr/lib/budgie-desktop/plugins/budgie-dropby/dropover
P: budgie-previews: executable-in-usr-lib usr/lib/budgie-previews/previews_controls
P: budgie-previews: executable-in-usr-lib usr/lib/budgie-previews/previews_creator
P: budgie-previews: executable-in-usr-lib usr/lib/budgie-previews/previews_daemon
P: budgie-previews: executable-in-usr-lib usr/lib/budgie-previews/previews_triggers
P: budgie-quickchar: executable-in-usr-lib usr/lib/quickchar/quickchar
P: budgie-showtime-applet: executable-in-usr-lib usr/lib/budgie-desktop/plugins/budgie-showtime/showtime_desktop
P: budgie-takeabreak-applet: executable-in-usr-lib usr/lib/budgie-desktop/plugins/budgie-takeabreak/message_window
P: budgie-takeabreak-applet: executable-in-usr-lib usr/lib/budgie-desktop/plugins/budgie-takeabreak/takeabreak_run
P: budgie-visualspace-applet: executable-in-usr-lib usr/lib/budgie-desktop/plugins/budgie-visualspace/auto_workspace
P: budgie-wallstreet: executable-in-usr-lib usr/lib/budgie-wallstreet/wallstreet
P: budgie-wallstreet: executable-in-usr-lib usr/lib/budgie-wallstreet/wallstreet_control
P: budgie-weathershow-applet: executable-in-usr-lib usr/lib/budgie-desktop/plugins/budgie-weathershow/desktop_weather
P: budgie-weathershow-applet: executable-in-usr-lib usr/lib/budgie-desktop/plugins/budgie-weathershow/get_color
P: budgie-window-mover-applet: executable-in-usr-lib usr/lib/budgie-desktop/plugins/budgie-wmover/moverbar
P: budgie-window-mover-applet: executable-in-usr-lib usr/lib/budgie-desktop/plugins/budgie-wmover/wmover_panelrunner
P: budgie-window-mover-applet: executable-in-usr-lib usr/lib/budgie-desktop/plugins/budgie-wmover/wmover_run
P: budgie-window-shuffler: executable-in-usr-lib usr/lib/budgie-window-shuffler/grid_all
P: budgie-window-shuffler: executable-in-usr-lib usr/lib/budgie-window-shuffler/gridwindow
P: budgie-window-shuffler: executable-in-usr-lib usr/lib/budgie-window-shuffler/jump
P: budgie-window-shuffler: executable-in-usr-lib usr/lib/budgie-window-shuffler/resize
P: budgie-window-shuffler: executable-in-usr-lib usr/lib/budgie-window-shuffler/shuffler_control
P: budgie-window-shuffler: executable-in-usr-lib usr/lib/budgie-window-shuffler/softmove
P: budgie-window-shuffler: executable-in-usr-lib usr/lib/budgie-window-shuffler/tile_active
P: budgie-window-shuffler: executable-in-usr-lib usr/lib/budgie-window-shuffler/togglegui
P: budgie-window-shuffler: executable-in-usr-lib usr/lib/budgie-window-shuffler/windowshufflerdaemon
P: budgie-workspace-wallpaper-applet: executable-in-usr-lib usr/lib/budgie-desktop/plugins/budgie-wswitcher/wswitcher_panelrunner
P: budgie-workspace-wallpaper-applet: executable-in-usr-lib usr/lib/budgie-desktop/plugins/budgie-wswitcher/wswitcher_run