The main goal of this refactor is to separate drawing the frame numbers from drawing the scrollbar in the timeline.
This is necessary for T63193.

Furthermore, this patch changes/fixes the position of some scale indicators in the UI.

Before:

After:

Simple test to see that I can draw frame numbers into arbitrary rectangles in the region now (not part of the patch):

Individual commits:

• move horizontal numbers drawing into separate function
• move vertical number drawing to separate function
• move declarations closer to first use
• improve variable naming
• use correct distance between numbers
• use correct start x location
• make number drawing independent of scrollers
• remove unused view2d units
• move variable definitions closer to their first use
• use less generic method for actually drawing the text
• draw text centered on grid lines
• fix text used for seconds
• separate vertical number drawing from scroller and position text correctly
• "better" special case for the sequencer
• only display text in a rectangle
• cleanup display-twice-avoidance
• rename "power" to "brevity_level"
• turn grid number drawing into api functions