Draft: Implement Send Voice Messages

WIP at this stage.

Design

Single click to open the recording popover. Click + hold to immediately start a recording.

When closing the popover, e.g. when clicking somewhere or switching the chat, the user is prompted to either send or not to send the attached recording file. In case the user decides not to send the file (cancel) the current approach is to start a new recording afterwards. The files remain on disk until Gajim quits.

Whenever a new recording is started it records into a new file called voice-message-date.m4a.part{i} in a temporary directory, where {i} is an increasing index for the number of resumed recordings. When the recording stops, all voice-message-date.m4a.part{i} files will be merged together into a final file.

The audio preview class has been extended to load audio files on demand after creating an instance. The audio visualizer got the ability to optionally not draw gaps between the two (possible identical) channels and to omit normalizing by the largest displayed peak as this looks visually better for the visualization of input volume over time.