Make rofi theme configurable

[twa022]

Release 22.04.0 hardcodes mate-hud to use the rofi docu theme which in my opinion makes it more difficult to use.
This changes the default to Arc-Dark (feel free to pick whatever you want) but also makes the rofi theme configurable in gsettings.

[vkareh]

Thanks for this change - it works very well. I would suggest keeping docu as the default theme, since that's the default intended by @flexiondotorg, and maybe adding a note that the user can run rofi-theme-selector as a way of testing out the various themes, instead of looking through the /usr/share directory.

[vkareh]

@twa022 - this PR just became very complex very quickly. I suggest sticking with the rofi-theme option, defaulted to 'docu', in a single commit. The rest sounds like it needs an extra PR or two separate from the theme selection.

[twa022]

@vkareh: fair enough. I reverted the commits that enabled additional configuration options and committed a change to set the default theme back to docu and to add instructions on how to change the theme in the README.

I didn't mention rofi-theme-selector in the notes since in this implementation it doesn't select the rofi-theme and I thought that could be confusing.

[twa022]

That said I do think it would be a good idea to reevaluate the default theme. You can see in the picture on #52 that with the docu theme you cannot actually tell what some of the menu items are as too much text is truncated to tell them apart. Perhaps the project could include its own rofi .rasi theme.

[vkareh]

I agree with the concern, but I'll let @flexiondotorg make that decision. I think adding a .rasi file specific to ubuntu-mate at some point makes good sense.

This new change works well for me, thanks! Can you squash your commits into a single one to clean up the history?

[twa022]

@vkareh: Done (I did not know you could do that, and my git histories will be much prettier now)

[vkareh]

Great, thanks!

LGTM

[flexiondotorg]

@twa022 Thank you for working on this. I'll get it packages and released ASAP.

I do agree that adding a .rasi file specific to Ubuntu MATE would be ace 🔥 I would welcome that addition 👍 Ubuntu MATE 22.04 is fast approaching final beta, but if you can supply a decent .rasi in the next couple of days, I'd really appreciate it. docu was the only theme in those shipped in Debian/Ubuntu that integrated the best.