vale: fix image alt text rules for wrapped alt text

QDoc allows alt text for \image on the next line, and for \inlineimage
too. The text must start with a brace. The old QDocImageAlt rule used
the noalt mark from the converter. The converter sets that mark when
nothing follows the file name on the same line. So the rule warned on
wrapped alt text.

Read the QDoc source instead, with scope raw. QDocImageAlt now warns
when an \image line holds only a file name and the next line does not
start with a brace. Alt text on the same line passes, with or without
braces. Turn off Vocab for the rule. A raw match over several words
never fires when the sentence has a Vocab term.

QDocImageAlt no longer checks \inlineimage. InlineImageAltBraces now
covers it. It warns when text follows the file name and does not start
with a brace. That text can be on the next line. It does not warn on a
bare \inlineimage.

Add tests for these cases:
- alt text on the same line
- braced alt text on the next line
- a blank line after the image
- unbraced text on the next line

Change-Id: Ie7e205bda0e9734d1517e8ab6800dfe1191197ec
Reviewed-by: Alexei Cazacov <alexei.cazacov@qt.io>