allow image widgets to do alpha-blitting

when the image (e.g. a PNG) does contain an alpha channel, that can
be honored by ImageWidget. It doesn't do so by default for compatibility
(arguably, we should change that in the future), it has to be enabled
by setting the "alpha" property to "true" (boolean, not string).
honor alpha when painting the dog-ear bookmark icon

allows for different icons making use of that. For our current icon,
it actually doesn't.

Fixes #402
Merge pull request #1295 from hwhw/master

fix handling of invalid UTF8 sequences
Merge pull request #1296 from hwhw/imagewidget_alpha

Imagewidget alpha
OPDS catalog: auto-fill http:// if no protocol given and revert #1106

This fixes #1259.
Merge pull request #1297 from Frenzie/language

OPDS catalog: auto-fill http:// if no protocol given and revert #1106
Prevent spaces from being eaten by Transifex

Fixes #1276
Merge pull request #1298 from Frenzie/master

Language: prevent spaces from disappearing in localizations with util.template
OPDS: ignore links with unknown rel values, treat empty rel as rel="subsection".
Merge pull request #1300 from erosennin/opds-ignore-unknown-rels

OPDS: ignore links with unknown rel values, treat empty rel as rel="subsection"
Add missing util.template definition to readerfrontlight.lua

Fixes #1302.
Merge pull request #1303 from Frenzie/language

Add missing util.template definition to readerfrontlight.lua
move refresh menu table out of UIManager

no need to clutter a core file of the whole UI concept with definitions
of UI elements
change setDirty/refresh API

See documentation in the code.
In short: There is now one single method, setDirty(), that triggers
repaints and/or refreshes.
All variables in UIManager are gone - at least from an external
perspective. Everything is done through setDirty().
This also allows for easier debugging, since all requests come
in via function calls.
adapt widgets to new refresh/repaint API
update base: use new refresh wait queue