Draft: Add underline/strikethrough buttons to text tool

This MR wires up the existing underline/strikethrough rendering and style processing code to two buttons in the text toolbar. Thanks @doctormo for some tips!

Partial fix for inbox#4983

underline

What works:

• Reading files that have text-decoration set, both as shorthand and individual properties (text-decoration-line etc.)
• Editing those files, the state of the new buttons is correct
• Adding underline in new texts (all text types)

What could be improved in this MR:

• Better icons for hicolor and tango (I just copied the multicolor one). @adam.belis , would you want to help me here?
• Can we edit the SPITextDecorationLine instead of recreating a CSS string in TextToolbar::decoration_changed? I didn't figure this out.

What is out of scope for this MR:

• Adding UI support for the other CSS3 decoration attributes (overline, linestyle, line color)
• There is currently a bug with the rendering that incorrectly "inherits" the text-decoration attribute. For <text style="text-decoration:red underline">Hello<tspan style="text-decoration:font-size:20px; text-decoration: blue line-through">World</tspan></text>, there should be a red (thinner) underline everywhere and a thicker blue strike-through for "World" due to the higher font size. text-decoration is never inherited. This MR exploits the bug to "un-underline" a part of an underlined text, by setting text-decoration-line: none on the newly-created child. With a proper rendering implementation, this would not work, and the file we created using this MR would not be "forward-compatible", and it would anyway not render correctly in browsers for this specific case.
• Text decoration is not taken into account for
  • PDF export (#952)
  • object to path (#956)
  • bbox and hitbox computation

The last two toplevel bullet points imo need to be fixed before releasing this (and @Tavmjong 's help would be much appreciated). If it's not fixed for 1.3, we should disable the feature (hide the buttons).