Lots of small miscellaneous changes and cleanup to how the Text Editor looks.

• Adds a theme item for the color of line numbers. This way the color can be toned down, or differ from text color. And also works better when using a light background.
• Removes all instances of placement and alignment involving non-scaled constants
• More padding around the line numbers and between them and the text.
• Multiple-line selection boxes no longer have an odd ragged left side.
• Text insertion caret is now scaled with the rest of the UI, so will be more readable on high-dpi displays like Retina. Also adjusted the placement of it, including in "insert" mode so it covers existing characters a bit less.
• The right "Margin line" is done in a way that works on light backgrounds.
• The "Highlight Line" option is one in a way that works on light backgrounds

With the current default theme, the following illustrates the changes in padding and alignment. Left is current, right is after patch is applied:

Following closeup (at 4X UI scale) shows changes to number padding and size and placement of cursor in "insert" mode:

Following shows how the dimming of the line numbers can help replicate the look of VSCode:

The following illustrates how the changes allow a theme designer to select a very light background, here looking like MSVS: