Use system colors

[dospuntos]

Fixes #48 - all windows are using color constants.
Fixes #23 - using default Haiku icons for alerts.

Additional changes:

• App menu with app icon (About, Quit, etc.)
• Alert modals are using default Haiku icons instead of the previous custom icons. The old icons did not work well with dark backgrounds.
• Alert modals are centered in main window when displayed. Enter a transaction without Payee or Category to see examples.

Notes:

• About-modal is also centered in main window, is this OK? Most native apps seem to center it on screen.
• A few sections in the main window (QuickTracker and Add Transaction form) will not live-update the text/background color when system colors are changed. Restarting the app brings in the right colors.

[humdingerb]

Something isn't right here. We go through all the effort to determine the right High- and LowColors, depending on selection, enabled and account-closed. Only to draw a rectangle.
Then in line 59 we potentially change the HighColor to stroke the frame.
Then we change the HighColor again in line 64 to draw the text.

I think the text drawing should be done right after filling the rect, when the colours to use are still valid.
Drawing the rect only needs the HighColor set. Maybe it can be simplified by using rgb_color variables and set them to either selected/unselected and then tint them if needed (enabled/disabled, account-closed).

This may be applicable to other drawing code as well...

Currently, the drawing isn't correct. Try using a white on blue text for selected list items:

[dospuntos]

@humdingerb I've updated the PR. Quite a lot of changes, but they are (almost) all related to the display colors.

I can't seem to get the code formatting right, even when running git-haiku-format?

[korli]

I can't seem to get the code formatting right, even when running git-haiku-format?

This isn't a problem, the code will be automatically formatted after the merge.

[humdingerb]

Thanks @dospuntos ! Weird that when changing system colours some things only appear right after restarting the app. Let's merge this now to get it off your plate. :)