Show possible compose completions on request

[mike-fabian]

When typing the key bound to enable_lookup (which is ['Tab', ISO_Left_Tab'] by default) during an incomplete compose sequence, show possible completions.

In order not to show insanely many possible completions, show only those which are possible to type on the current keyboard layout.

[mike-fabian]

I implemented something which looks like this:

https://user-images.githubusercontent.com/2330175/128645885-6bd2c39c-2055-4690-8dab-dff05b01f27e.mp4

[mike-fabian]

Here I am using a Greek keyboard layout and type dead_grave Tab. In the possible completions shown are sequences like

` α      ὰ U+1F70 greek small letter alpha with varia

where one has to type Greek letters like α. These are only shown because the Greek keyboard layout has these letters. When typing dead_grave Tab for example on the US English international layout, these completions using Greek letters would not be shown.

When using a Russian layout, completions using Cyrillic letters are shown.

This is to reduce the number of possible completions shown, only completions for sequences which can actually be typed on the current keyboard layout are shown .

https://user-images.githubusercontent.com/2330175/128646020-ed2f7b54-09b4-480e-a476-a56a66f583a3.mp4