gui2/progress_bar: Modernize appearance

The old design was heavily based on the old GUI0 loadscreen, inheriting
the weird blocky 3D shading effect reminiscent of the old Windows 9x
times. This makes it look very obviously out of place with the current
Wesnoth UI look and feel.

We increase the default width of the progress bar to reduce visual gaps
between different phases of the add-ons client's initial connection
sequence, and decrease the height to something that is more in
proportion with the font size without taking up more space than
necessary. The colours are inspired by the current textbox and button
widgets while keeping consistency with the rest of the game's UI.

[ci skip]

data/gui/widget/progress_bar_default.cfg

@@ -11,10 +11,10 @@
 	[resolution]
 
 		min_width = 14
-		min_height = 41
+		min_height = 36
 
-		default_width = 54
-		default_height = 41
+		default_width = 400
+		default_height = 38
 
 		max_width = 0
 		max_height = 0
@@ -23,55 +23,30 @@
 
 			[draw]
 
-				# Outer "golden" border.
 				[rectangle]
 					x = 0
 					y = 0
 					w = "(width)"
 					h = "(height)"
 					border_thickness = 1
-					border_color = "188, 176, 136, 255"
+					border_color = {GUI__BORDER_COLOR_DARK}
 				[/rectangle]
 
-				# Inner black border, with gray filling.
 				[rectangle]
 					x = 1
 					y = 1
 					w = "(width - 2)"
 					h = "(height - 2)"
-					border_thickness = 1
-					border_color = "0, 0, 0, 255"
 					fill_color = {GUI__BACKGROUND_COLOR_ENABLED}
 				[/rectangle]
 
-				# 3D effect at upper side.
 				[rectangle]
 					x = 2
 					y = 2
 					w = "(((width - 4) * percentage) / 100)"
-					h = 2
-					border_thickness = 0
-					fill_color = "79, 103, 123, 255"
-				[/rectangle]
-
-				# Main bar
-				[rectangle]
-					x = 2
-					y = 4
-					w = "(((width - 4) * percentage) / 100)"
-					h = "(height - 8)"
-					border_thickness = 0
-					fill_color = "21, 53, 80, 255"
-				[/rectangle]
-
-				# 3D effect at lower side.
-				[rectangle]
-					x = 2
-					y = 37
-					w = "(((width - 4) * percentage) / 100)"
-					h = 2
+					h = "(height - 4)"
 					border_thickness = 0
-					fill_color = "10, 26, 40, 255"
+					fill_color = "0, 55, 82, 255"
 				[/rectangle]
 
 			[/draw]