Plugin Manager dialog size incorrect when download progress dialog shown first

[johann-petrak]

The plugin manager window normally gets sized to a proper size by default, which allows to view all parts of the dialog.

However, when the plugin cache gets refreshed, a small download progress dialog window is shown first and after this, the plugin manager dialog keeps the same size, which is too tiny for the plugin manager dialog.

Low priority and cosmetic but should not be too hard to fix.