Fix bob widget issues

[abigailalexander]

This PR fixes various issues with widgets that I noticed when using .bob files:

• Added image_stretch field (this is stretch_to_fit in .opi files)
• Modified bobParseFonts to use strings instead of numbers as keys
• Added a check for whether a .opi or .bob file is used for rules, as .opi uses pv_name and .bob uses pv
• Changed default background and text colour for Label and Readback to match with Phoebus defaults