Minor LMMS issues and design flaws

Nobody has claimed this yet.

Here's 3 things i recently noticed

#### Reset function is broken on text-based controls
As i stated in #5261 here, thinking it was due to the PR
https://github.com/LMMS/lmms/pull/5261#issuecomment-839668463
The reset function there isn't properly working.
It just proposes again the number that is already there.

You can clearly test this on your LMMS copy, for BPM and Time signature.

So, it should either works, or not be there if it's not supposed to work with those.


#### Subwindows flickering
I'm not sure about this problem, as i couldn't properly record it, but it's there, at least on my laptop on the 60hz panel it has.
So basically part of the windows (as song editor, piannoroll ecc) gets cut out when you move it in a direction.

To reproduce it, just take a windows of your liking, for examples song editor and keep it not maximized so that you can move it.
Now, assuming you're using the default theme but should be the same on every theme, just move it with a decent speed towards the right and you will see the light 1px border on the right getting cropped out and reappearing when you stop.
The same goes for every border, except the top one which has the title bar.

I'm not sure if it's QT related but it's not something about my laptop as it doesn't happen for example with windows explorer windows.

I made a mockup


#### Minimum Mixer
This is more of a flaw from a design perspective, than an issue.
The mixer window can't be reduced more than this


And i'm not sure why about this.

Most importantly if it really can't be resized to be less than that, i think it should at least have some mixers preloaded there to fill the space. A couple of empty mixers won't affect performances anyway.


Leaving it like that would just leave a big emptyness.

I know one could make a custom template, but i haven't seen yet a daw that starts with just the master mixer, apart from mulab, but that one adds a new mixer everytime you add an instrument. So maybe the default template should be changed, i don't know.

Anyway there's no reason why it can't be resized less if it starts with just no additional mixers by default.

Both the mixer thing and the subwindow one aren't a big deal if LMMS will ever move to a singlewindow design, but we all know that would take a lot of time.

#### Affected LMMS versions
LMMS 1.2.0 Stable, but i believe also 1.3.0 and more recent stable liek 1.2.2

OS: Windows 10 64-bit

The issue bundles a struck-out reset problem with reports about subwindow flickering and the mixer’s minimum size. Start by reproducing the remaining two behaviors on LMMS 1.2.x under Windows 10, then trace the affected desktop-window and mixer UI code. Done means each behavior has a confirmed scope and an agreed fix or documented design decision, with regression coverage where applicable.

Written by the indexing model from the issue text.