wip: signs

[alexekdahl]

Resolves: #27

[fadedreams]

I just tested your PR #28 with the sign column feature — it works perfectly! The glyphs show up nicely in the sign column, and everything feels snappy. Great addition!
One small UI suggestion for the Show marks window:
Right now, the file names are misaligned because of the ● symbol:

[1] a2_2                 a2
[2] a2_5                 a2
●[3] var_jwt              j.js
●[4] a1_1                 a1

It would look much cleaner if the file column was aligned with padding, like this:

 [1] a2_2			  a2
 [2] a2_5		      a2
●[3] var_jwt		  j.js
●[4] a1_1		      a1