Saving .pxo files in HTML5 is now possible

Addresses #280

src/UI/Dialogs/ExportDialog.tscn

@@ -346,8 +346,8 @@ window_title = "Open a Directory"
 resizable = true
 mode = 2
 access = 2
-current_dir = "C:/Users"
-current_path = "C:/Users/"
+current_dir = "/Users"
+current_path = "/Users/"
 
 [node name="PathValidationAlert" type="AcceptDialog" parent="Popups"]
 margin_left = 8.0

src/UI/Dialogs/OpenSprite.tscn

@@ -8,5 +8,5 @@ resizable = true
 mode = 0
 access = 2
 filters = PoolStringArray( "*.pxo ; Pixelorama Project", "*.png ; PNG Image", "*.bmp ; BMP Image", "*.hdr ; Radiance HDR Image", "*.jpg,*.jpeg ; JPEG Image", "*.svg ; SVG Image", "*.tga ; TGA Image", "*.webp ; WebP Image" )
-current_dir = "C:/Users"
-current_path = "C:/Users/"
+current_dir = "/Users"
+current_path = "/Users/"

src/UI/Dialogs/SaveSprite.tscn

@@ -12,5 +12,5 @@ margin_bottom = 48.0
 resizable = true
 access = 2
 filters = PoolStringArray( "*.pxo ; Pixelorama Project" )
-current_dir = "C:/Users"
-current_path = "C:/Users/"
+current_dir = "/Users"
+current_path = "/Users/"

src/UI/Dialogs/SaveSpriteHTML5.tscn

@@ -0,0 +1,33 @@
+[gd_scene format=2]
+
+[node name="SaveSpriteHTML5" type="ConfirmationDialog"]
+visible = true
+margin_right = 200.0
+margin_bottom = 70.0
+
+[node name="FileNameContainer" type="HBoxContainer" parent="."]
+anchor_right = 1.0
+margin_bottom = 41.0
+__meta__ = {
+"_edit_use_anchors_": false
+}
+
+[node name="FileNameLabel" type="Label" parent="FileNameContainer"]
+margin_top = 13.0
+margin_right = 67.0
+margin_bottom = 27.0
+text = "File name:"
+__meta__ = {
+"_edit_use_anchors_": false
+}
+
+[node name="FileNameLineEdit" type="LineEdit" parent="FileNameContainer"]
+margin_left = 71.0
+margin_top = 8.0
+margin_right = 200.0
+margin_bottom = 32.0
+size_flags_horizontal = 3
+size_flags_vertical = 4
+__meta__ = {
+"_edit_use_anchors_": false
+}