Fix #86, #82, #241, #233

[maoschanz]

Sorry i read CONTRIBUTING.md too late, so i will give more details about what i did:

Use the system-wide setting concerning the appmenu

Issue #233, people who can't have or don't want the appmenu will not have it automatically. Files changed are:

• data/com.github.fabiocolacio.marker.gschema.xml
• src/marker-prefs.c
• src/resources/ui/marker-prefs-window.ui
• src/marker.c

Translations

Issues #82 and #241, all the gettext thing should work, i also provided the french translation. Files changed are:

• some meson.build files
• everything in po
• src/marker.c
• src/marker-editor.c
• almost everything in src/resources/ui/
• the bash script is here just because i find it convenient

Help

Issue #86, but i'm not a native english speaker and i don't use Mermaid and Charter so you might like to write more accurate and complete help pages. Files changed are:

• some meson.build files
• everything in help
• src/marker.c
• menus in src/resources/ui/
• i moved the screenshots to the help directory so they could be included in the help pages, so i changed README.md too

IMO, screenshots with more little windows should be used, currently it's hard to choose between huge pictures and tiny characters

Other things

• The layout of the primary menu when the appmenu is disabled was not very nice, i changed the order of items in it (src/resources/ui/marker-gear-popover.ui and src/marker-editor.c)
• My changes to src/resources/ui/marker-gear-popover.ui should conflict with what @Exalm did concerning the "About" item
• I changed the spacing and the packing in src/resources/ui/marker-prefs-window.ui because everything was very compact
• In the menu renamed "View" to "View mode", and all items "something mode" to just "something"

[fabiocolacio]

Hi @maoschanz,

Thanks for you contributions, these are all great, and this fixes some long-standing issues with Marker. I cannot overstate how appreciative I am.

With that being said, please break this up into separate pull requests for each feature. That makes it much easier for me to test each feature separately, and request changes to them independently from each other as needed.

Please submit the Appmenu, Translations, Help, and the other UI changes as 4 separate pull requests.