With the Material Design icons, any other icons brought in through
the system icon theme will look inconsistent.
Also remove the KEEPASSXC_IGNORE_ICON_THEME environment variable
(which was introduced during development of the new icons to
disable the system icon theme and is thus no longer needed).
Fixes#475
Use the following to run KeePassXC with the icons from the
source code, ignoring the operating system's Qt icon theme:
```
KEEPASSXC_IGNORE_ICON_THEME=1 keepassxc
```
The patch further adds a script `makeicons.sh` that re-creates KeePassXC
icons from the Material Design icon set and can be used for easily
updating icons in the future. Instructions are in the script.
Fixes#475
KeepassXC tries to load the theme icon first and then falls back to the internal icon unless the check is explicitely disabled. Remove the check from most icons
Fixes#756
