feature request: Specify initial geometry in terms of gravity or margins

# Description of the new feature/enhancement

Extend the initial geometry settings fields with options that allow setting the terminal window to some percentage of the screen and position relative to the screen. For example width and height as a percent, together with left/right/top/bottom margins as either pixels or percents. Some of these options conflict with each other, so it might be good to mimick say CSS resolution rules.

This would allow a single configuration to be used on multiple resoltions.

No files, tests, or entry points are identified in the issue, so start by locating the initial geometry settings implementation. Define how percentage-based width and height combine with pixel or percentage margins, including conflict resolution, and verify that one configuration positions and sizes the terminal across screen resolutions.

Written by the indexing model from the issue text.