Add colormap combobox and utils (#195)

* feat: add colormap combobox

* working on styles

* add comment

* style: [pre-commit.ci] auto fixes [...]

* progress on combo

* style: [pre-commit.ci] auto fixes [...]

* decent styles

* move stuff around

* adding tests

* add numpy for tests

* add cmap to tests

* fix type

* fix for pyqt

* remove topointf

* better  lineedit styles

* better add colormap

* increate linux atol

* cast to int

* more tests

* tests

* try fix

* try fix test

* again

* skip pyside

* test import

* fix lineedit

* add checkerboard for transparency

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

examples/colormap_combo_box.py

@@ -0,0 +1,19 @@
+from qtpy.QtWidgets import QApplication, QVBoxLayout, QWidget
+
+from superqt.cmap import CmapCatalogComboBox, QColormapComboBox
+
+app = QApplication([])
+
+wdg = QWidget()
+layout = QVBoxLayout(wdg)
+
+catalog_combo = CmapCatalogComboBox(interpolation="linear")
+
+selected_cmap_combo = QColormapComboBox(allow_user_colormaps=True)
+selected_cmap_combo.addColormaps(["viridis", "plasma", "magma", "inferno", "turbo"])
+
+layout.addWidget(catalog_combo)
+layout.addWidget(selected_cmap_combo)
+
+wdg.show()
+app.exec()