Improve custom widget styling

Add a shared styling priority and move the custom styling to resource files.

added 2 commits

• 9696e84a - expander-row: Move the custom style to a resource
• a1497c3e - combo-row: Move the custom style to a resource

Compare with previous version

added 7 commits

• a1497c3e...5edf3070 - 4 commits from branch Librem5:master
• 6bd9950e - Add HDY_STYLE_PROVIDER_PRIORITY
• b85242f6 - expander-row: Move the custom style to a resource
• 747e3e99 - combo-row: Move the custom style to a resource

Compare with previous version

Ping #94 (closed)

merged

I got confused by the MRs topic since I thought it would improve the styling but it rather improves how we handle styling in libhandy...anyways, merged now.