Show Tile swap's Swap rate as a slider, and let it go slower

A new ParamSpec::slider() marks an Int or Double param to be shown in
the interface as a Gtk::Scale with its minimum and maximum marked under
the ends, rather than a spin button; the command line and the project
file are unchanged. Swap rate uses it, and now runs from 0 to 5 in
steps of 0.01 (was 0 to 30 in steps of 0.1): with every tile swapping
on its own, even 0.1 swapped the canvas many times a second.

Version 0.8.2.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/core/params.cpp

@@ -16,6 +16,12 @@ ParamSpec ParamSpec::styled() const {
     return copy;
 }
 
+ParamSpec ParamSpec::slider() const {
+    ParamSpec copy = *this;
+    copy.onSlider = true;
+    return copy;
+}
+
 ParamSpec ParamSpec::makeInt(std::string key, std::string label, long defaultValue, long minimum,
                              long maximum, std::string help, long step) {
     ParamSpec spec;