Fixes compiler warnings.

Sets strict compiler flags only for non-generated non-external sources.

Removes "-Werror" compiler flag so that compilation doesn't stop after a
warning. The flag makes development very difficult. Occasionally, it's a
good idea to check CI build output -- GitHub Actions list the warnings
very well.

Merges fonts to single file.

Removes usage of QFont::insertSubstitutions() which fixes following
warning in Qt 5.15:

    QtWarning: Populating font family aliases took 1523 ms. Replace uses
    of missing font family "CopyQ Icon Font" with one that exists to
    avoid this cost.

Signed-off-by: Lukas Holecek <hluk@email.cz>

Fixes: #1369
Signed-off-by: Lukas Holecek <hluk@email.cz>

Signed-off-by: Lukas Holecek <hluk@email.cz>

Signed-off-by: Lukas Holecek <hluk@email.cz>

Script to update icon font also adds a suffix to family name so won't
conflict with font installed on system (which could have incorrect
version).

Adds support for icon search. This saves a lot of time since user
doesn't have to search visually through all icons. User just types some
text (search terms are retrieved from font's metadata; like "clipboard",
"left", "next" etc.) and non-matching icons will be hidden.

In icon selection dialog, brand icons are at the end of list and have
slightly different background color.

Few icons are missing.