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/effects/tileswap.cpp

@@ -25,10 +25,11 @@ const std::vector<ParamSpec>& TileSwapEffect::params() const {
                            "How many tiles are dropped on the canvas; scattered layout only."),
         ParamSpec::makeBool("tileswap_keep_order", "Cycle images in order", false,
                             "Take tiles from the images in order instead of picking randomly."),
-        ParamSpec::makeDouble("tileswap_rate", "Swap rate", 1.0, 0.0, 30.0,
+        ParamSpec::makeDouble("tileswap_rate", "Swap rate", 1.0, 0.0, 5.0,
                               "Swaps per second of each tile, rounded so that the clip loops "
                               "without a jump; 0 for none.",
-                              0.1, 1),
+                              0.01, 2)
+            .slider(),
     };
     return specs;
 }