Redesign Pluma Preferences Dialog

[mbkma]

Reason for PR: Preferences Dialog in master is very tall with the new options. I imagine this to be problematic on small displays.

• Group text appearance preferences together in order to make dialog less tall.
• Remove empty labels and use widget margins instead.
• In "Show Spaces,...": Indent subitems a little more
• Indent "Do not split words.." box in "Text Wrapping" (Because its a subitem. I am not sure about this one though.)
• Do not indent box with colour schemes
• Capitalize headings (Grid pattern -> Grid Pattern and Show spaces, tabs, newlines -> Show Spaces, Tabs, Newlines). See https://headlinecapitalization.com/

Edit: The following additional changes were done:

• Group options into Display and Highlighting categories
• Fix sensitivity of auto-save spinbutton
• Display right margin with spinbutton in one line
• Apply sensitivity on spinbutton directly instead of according ui objects "autosave_hbox" and "right_margin_position_hbox"
• Adjust order of "Spaces, Tabs, Newlines" checkboxes to match the headings order

[rbuj]

I agree with everything except the first group of options Text Appearance, which groups two highlighting options with displaying the line number.

Before

After

[mbkma]

another suggestion:

[rbuj]

another suggestion:

Yes, it looks better now.

[mbkma]

@rbuj thanks for the review, I think it is fixed now

[mbkma]

Just fixed an a11y issue and reworded commit message.