(cherry picked from commit 6eb75b82)

Igor Poboiko authored May 31, 2020 and Laurent Montel committed May 31, 2020

Rescale image only when needed (i.e. original image is larger than
desired). Also, when user don't want to rescale the image at all,
special values  width == -1 && height == -1 are provided; this case
should also be covered.

The width and height are currently used to set properties of
QTextImageFormat, which then are used to set "width" and "height" html
properties for img tag, so that rescaling will be done by whoever reads
generated html. The image itself is not affected.
By also rescaling the image itself, we can save some space and reduce
the size of the document
(e.g.: I tried to add a HD photo of original size 4000x2000, whose base64
was ~12mb; I resized it to 500x250, and the size reduced to 300kb)

activateRichText emits textModeChanged signal, which leads to
richTextComposerActions::textModeChanged call. Which segfaults if table
actions are not yet created by explicit createActions call.

The old code doesn't set the flag richTextEnabled, which results in
some actions being enabled and some being disabled due to checks in
slotUpdateMiscActions.