Use posix spawn in Sys::execute when available and possible.

Add jEdit color scheme (SpartanJ/ecode#660).
Add a `--first-instance` CLI option to open files in the oldest ecode instance (SpartanJ/ecode#659).

bin/assets/colorschemes/colorschemes.conf

@@ -711,3 +711,45 @@ operator = #000000
 function = #000080,bold
 link = #007f00
 link_hover = #007f00,transparent,underline
+
+[jEdit]
+background = #ffffff
+text = #000000
+caret = #000000
+selection = #ccccff
+line_highlight = #ffffe0
+line_number = #464646
+line_number2 = #181818
+gutter_background = #dfdfdf
+whitespace = #c0c0c0
+line_break_column = #54575b99
+matching_bracket = #dbdbdb
+matching_selection = #ffff00
+matching_search = #0000f0
+suggestion = #1d1f27,#e1e1e6
+suggestion_selected = #222533,#ffffff
+suggestion_scrollbar = #3daee9
+error = #ff3030
+warning = yellow
+notice = #8abdff
+selection_region = #c0c0c077
+minimap_background = #FFFFFF1A
+minimap_visible_area = #dfdfdfAA
+minimap_current_line = #f0f0f00A
+minimap_hover = #FFFFFF1A
+minimap_selection = #8abdff80
+minimap_highlight = #FFFF0080
+
+normal = #000000
+symbol = #000000
+comment = #cc0000
+keyword = #006699,bold
+type = #0099ff,bold
+parameter = #009966,bold
+number = #ff0000
+literal = #cc00cc
+string = #ff00cc
+operator = #000000,bold
+function = #9966ff
+link = #0000ff
+link_hover = #0000ff,transparent,underline