Change of image sizing unit

[AnonymouX47]

So far, image sizes have been in some form of "pixels" based on the style of rendering being used (i.e two pixels per character cell) but I recently realized how bad a design choice that was.

Plans are in place to add more render styles and the current size unit system will not make sense across the different styles.

Image sizes will be changed to be in units of columns and lines which is applicable to any render style as long as it's in a terminal.

Progress:

• Modify TermImage.
• Update TUI urwid widgets.
• Remove render * and update terminology everywhere.