Improve i3 support, and generalize desktop support in the process

[dannyedel]

Motivation: I use it myself

What should it do

• Query RANDR outputs via i3 ipc
  • i3-msg -t get_outputs gives JSON
  • Only check active:true and hope it's two of them
  • The one with primary will likely be the laptop screen
• Use i3's IPC mechanism to send "Move window id to output name

This can be generalized to "Desktop environment support modules", and move the Window-Movement code away from the PDFViewerWindow class.

DE Support Modules should support the following functions:

• Is this my environment?
  • Outputs unsigned value of how good the module is for the current DE
  • 0 this is definitly not my environment. Do not call other functions on this handler
  • 1 for the generic handler, meaning the current X11 handler
  • 10 if the OS matches (move the current WIN32 handler here)
  • 100 or for specific handlers that can detect their special environment is active (such as an i3 handler)
• Enumerate outputs
  • Returns vector of unique_ptr to handler-specific structure
• Get primary output
  • Can throw "Cannot figure out which is the primary"
  • If exception ignored, picks the numerically first
• Get secondary output (parametrize to ignore errors)
  • Can throw "Only one screen"
  • If not called to ignore errors, will throw "Cannot figure out which is the secondary"
  • If not called to ignore errors, will throw "Too many screens"
  • If errors ignored, outputs second screen
• Move window to output $handle

Make sure the interface is clean, that is near-impossible to use incorrectly. (No raw pointers etc.)

[clawoflight]

May I question why this is here? It probably makes sense for other DEs, but for i3 specifically, this is trivial to do:

I simply assign workspace 10 to always be on the output I use for presentations, and assign the dspdfviewer window to always be on that workspace. Job done, literally 2 lines, one of which I had already before using dspdfviewer. Has the nice advantage that it doesn't break if I open dspdfviewer before connecting to the projector :)

workspace $ws10 output DP2 # VGA output
assign [title="DS PDF Viewer - Audience Window"] $ws10