Allow to set first line indent for paragraphs

[jaap-karssenberg]

See https://bugs.launchpad.net/zim/+bug/1463309. Similar request from users for "creative" writing.

Implementation should use TextView.set_indent() to set default paragraph indent. But also update at least tags for headings to overrule this default. Add to styles.conf and in preferences dialog.

Will not look good when using many short lines, but works for long paragraphs without line breaks. Default set to zero, allow users to choose if they want this.

[LazyJ]

I realize Zim wiki's purpose is not to be a word processor but a few word processor visual features could be some quality of life improvements.

Having a format toggle that automatically indents the first line of paragraphs and being able to toggle left and right margins on/off come to mind for notes that have a lot of multi-line paragraph text.

Perhaps these features could be implemented as an plug-in?

Anyway, casting my vote in the hat for a first line indent feature.

Thanks for creating Zim. It's my daily workhorse.