Pretty neat (Although Qt guis tend to make me sad), a few things I've noticed with quick testing:
It would be neat if you could set up a table based highlighter. Since I use bass, things look pretty weird on my end. Also, when I add bit widths to my opcodes, they get interpretted as labels and so are a different color and I no longer get the tooltips while hovering on those opcodes.
In that same vein, it would be nice to add regex as a possibility for highlighting as well, That way we could highlight things like "#$xx" and "$xx" without having to add a new rule for each possible value. It would also be good to have multiple classes of opcodes, so not all opcodes are the same colors, so that you can differentiate them.
I don't know if this is you or my system, but the font defaulted to Sans Serif, which tends to be a horrific font for coding. It should probably default to the system monospaced font.
It looks like you probably used static positioning and size for your widgets, which makes it look like this on my system:
I'm not entirely sure how Qt handles it, but there should be an equivalent to Gtk's H/VBox and grids that would allow them to grow to fit content. Also the windows are static size, which is a very Java thing to do. If you use boxes/grids (layouters? layouts? whatever they are called in Qt) you should be able to allow the windows to grow to as large as the user wants them. This was a big problem in the old status effect, which you could see any time you tried to run it without using the default windows XP theme.
When I press load project, it immediately closes all my tabs, so even if I press cancel, I have to go through and reopen them all. I suspect that is not desired behavior.
Snippets don't seem to respect indentation. Rather than using the user's indentation level, it just uses it's own, which ends up making it look like this:
;AXY = 8-bit.
!Type = $1747 ;$1747 = Extended sprites,
;$B6 = Regular sprites.
;$182C = Minor extended sprites.
STZ $00 ;initialize scratch RAM
+ LDA #$10
STA !Type,x ;Store into somespritetype's X speed
which I then have to fix. If you would make it detect the Whitespace before the cursor and just insert that before each line, I suspect that would make it much easier.
A default tab width of 12 is pretty odd. I would go with either 4 or 8. It would also be nice to have an option to replace tabs with spaces when the user presses the tab button.
When I change the file and ctrl+z all the way back to the beginning, it still says the file has been edited. Not a big deal, but it would be nice to change it back to not edited if it returns to it's original state.
All the templates seem to be connected to the same bit of code. No matter which one I try to insert, it just inserts the code I posted above.
Erm... I think that's it. Shaping up pretty nicely, though I don't know if it can sway me from Sublime Text.