Don't draw frames and shadows around images with transparency

Summary:
This patch improves and simplifies the criteria for whether or not to draw a frame and a shadow around an image's thumbnail. The old size-based detection code was unreliable and gave false positives as well as false negatives. It is replaced with a simple check for whether the image has an alpha channel, which not only automatically matches all icon files, but also non-icon raster images with transparency, which look better without the frame.

BUG: 258514
FIXED-IN: 5.50

Test Plan:
== PNG with no transparency ==
Previously most would get frames, but some wouldn't, depending on their dimensions. Now all do.

Before:
{F6215116}

After:
{F6215118}

== PNG with transparency ==
Previously most would get frames, but some wouldn't, depending on their dimensions. There was also a bug where screenshots that includes shadows got a double shadow (one from the image itself, another from the frame-and-shadow. Now none get the frame.

Before:
{F6215109}

After:
{F6215110}

Before:
{F6214261}

After:
{F6214262}

Before:
{F6214955}

After:
{F6214956}

Before:
{F6214259}

After:
{F6214260}

Before:
{F6214264}

After:
{F6214265}

== JPEG ==
Previously most would get frames, but some wouldn't, depending on their dimensions. Now all do.

Before:
{F6214939}

After:
{F6214940}

== SVG ==
Previously most (but not all) icon SVGs would not get frames, but some would, depending on their dimensions. Non-icon SVGs would mostly get frames, but some would not, depending on their dimension. Now they only do if they have transparency.

Before:
{F6214943}

After:
{F6214944}

Before:
{F6214977}

After:
{F6214968}

Reviewers: #frameworks, #dolphin, #vdg, broulik, cfeck, abetts

Reviewed By: #vdg, broulik, abetts

Subscribers: anthonyfieroni, markg, abetts, bruns, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D15071