Fix seek preview thumbnails exceeding UI dimensions

[felix-hoc]

Description

Fix seek preview thumbnails exceeding the UI dimensions.

Fixes #569

Changes

• set the seekPreview position inside of the label, and pass the value in px (instead of %)
• check if the seekprevew label overflows the UI, and if so, limit its movement
• add a padding around the seekbarlabel wrapper, to keep the thumbnail a certain distance from the UI edges
• use span element instead of ::after for caret. This was required to "fix" the caret position to the cursor

Note: These changes cause a "Forced Reflow" (layout re-evaluation) when seek preview is active. However, the performance impact seems to be minimal, as only a small part of the layout tree needs to be re-evaluated.

Checklist (for PR submitter and reviewers)

• CHANGELOG entry