main/postmarketos-ui-mate: improve touch-friendliness

Maintenance:

• Changed version numbering to SemVer type.
• Renamed gschema override files for "system background" and "screensaver" to prefix number "10", that's more common for distribution-delivered gschema override files. Basically the prefix number just separates the override files from the xml files.
• For gschema override files "caja single click" and "mate terminal colors" replaced underline by dash character for better alphabetic sorting.
• In the APKBUILD, adapted the install of file "gtk.css" to the "-t" option, as no file name change is needed.
• Sorted the installs alphabetically per path, except the for the file "61-lightdm-autologin.conf", this one is at the end because it doesn't use the "-t" option due to file renaming.

Touch-friendliness:

• Increased size of the upper and lower panel from 24 to 36 px. This also increases the size of the icons in the panel. Unfortunately, at installation the panel layout config file is stronger than the gsettings override of the panel size, therefore a new panel layout config file is needed. It corresponds to the upstream default layout, just the panel size values were changed.
• Increased the size of the window buttons by changing the window border theme to "HighContrast". That's not the nicest looking theme but the one with the largest window buttons – convenient for touch devices.
• Increased general font sizes from 10 to 12 px. This goes along with increased panel size and window button size, avoiding looking it too unproportional.
• Changed the layout of the panel clock to just show the time (no date or week day). This saves space on devices with small screen. As the panel size was increased, display size gets tight on devices with small screen.
• Changed the layout of the panel workspace switcher to show names in the format of "- x -". This is a workaround to artificially increase the size of the workspace switcher buttons on portrait displays. Because otherwise these buttons are very squeezed on portrait displays due to the display ratio.
• Add an application shortcut for a maximized terminal to the users Desktop upon installation.

In case someone intends to test this MR on qemu: On default qemu resolution 640x480 the "Apply" button to change resolution in the MATE settings is out of the screen. I submitted MR !5712 (merged) to mitigate this. Until then the resolution can by changed by xrandr: sudo apk add xrandr and xrandr --size 800x600 (via ssh with a prepending DISPLAY=:0).