gui2/tslider: Add minimal variant without a text label

The default variant's text label is both inflexible in content (can't
customize the format for non-edge values) and layout (reserves way more
space than it needs at a time for its text label, often resulting in
huge horizontal gaps between a slider and an adjacent widget). This
makes it unsuitable for use in gui2::tchat_log.

This new minimal variant will be wired into gui2::tchat_log next.
gui2/tchat_log: Use a separate proper label for the page number text

This replaces the page number slider with a "minimal" variant slider and
an adjacent label widget whose text we enter manually.

The label format is "<PAGE>/<COUNT>" and it's untranslatable. Hopefully
this shouldn't cause any localization issues since it's all numbers and
we currently can't use alternative numeral systems anyway.

I'm leaving the page slider's maximum_value_label attribute intact just
in case this needs to be reverted later, since it's going into the 1.12
branch too.