Add support for more keys

[vojtapl]

Adds support for some keys, which would be required for a standard keyboard layout. Some notes:

• The tab and arrow keys currently use labels, but icons may be better (I used labels as I was unable to find a list of icon names), especially for the tab key.
• My comments in src/lib/models/key.h are a bit lackluster.

[compilebunny]

I'm not a developer on this project, but this sounds like a great addition. Tons of people are trying to figure out how to get maliit to provide a keyboard that is appropriate for a terminal.

[vojtapl]

I should have mentioned my motivation - I wanted to create a desktop terminal layout, but got stuck here #229. These keys seemed mature enough for a draft. I may close it, if you think I opened it too soon.

[ell1e]

For using the terminal, tab is really needed so this is promising. How does this draft look like, does anybody have a screenshot? Are the keys still large enough to have good accessibility for anybody typing impaired?

[vojtapl]

@ell1e Here is a screenshot of the keys with current symbols added to a new row (some changes may be needed for these). Only two of the arrow keys are there, as all of them did not fit nicely. Code for the new row:

Row {
      anchors.horizontalCenter: parent.horizontalCenter;
      spacing: 0

      DownKey {}
      RightKey {}
      CapsLockKey {}
      EscapeKey {}
      HomeKey {}
      EndKey {}
      PageUpKey {}
      PageDownKey {}
      TabKey {}
}

[ell1e]

Thank you so much! Sadly, your screenshot confirms my fears. Given how vertically cramped the keys already are on some devices and how much space the keyboard takes up, an additional row seems to me like it's possibly a really bad idea:

Wouldn't moving all those new keys, like escape and the arrows, to a new pane also solve that not all the arrow keys fit? I think it would make more sense to add more panes to the bottom-left special pane button to cycle through, and add the terminal keys there.

[vojtapl]

@ell1e Oh, that layout was just quickly cobbled together for that image. Here is how it can look when slightly better arranged for a larger screen :). In regards to the layout switching I think it sounds like a great idea, but I do not think I can pull it off with my current knowledge and time available.