Fix UI tooltips

[psykana]

Radio button, checkbox, slider gadgets set their tooltips here:

Thyme/src/game/client/gui/gamewindowmanager.cpp

Lines 769 to 774 in b7a373a

	if (tooltip_window->m_tooltip != nullptr) {
	tooltip_window->m_tooltip(tooltip_window, &tooltip_window->m_instData, packed_mouse_coords);
	} else if (tooltip_window->m_instData.Get_Tooltip_Text_Length() != 0) {
	g_theMouse->Set_Cursor_Tooltip(
	tooltip_window->m_instData.Get_Tooltip_Text(), tooltip_window->m_instData.m_tooltipDelay, 0, 1.0f);
	}

Since GameWindow ctor incorrectly initialized window->m_tooltip to a stub function, Thyme never got past the first check.

That didn't fix comboboxes though, which led me to GameWindowManager::Is_Hidden().

Fixes #1138
Fixes #998

[xezon]

Changes in GameWindowManager::Is_Hidden look logical to me. Change in constructor I do not know.

[jonwil]

Changes in GameWindowManager::Is_Hidden are correct.

[jonwil]

Looks good to me, approved.