MIPS relocation pairing, '$' prefix option & fixes

Implements MIPS relocation pairing logic.
New option for "Register '$' prefix", off by default.
Fixes various regressions introduced by refactoring.

Resolves #122
Resolves #156

objdiff-gui/src/app_config.rs

@@ -273,7 +273,6 @@ impl DiffObjConfigV1 {
             },
             space_between_args: self.space_between_args,
             combine_data_sections: self.combine_data_sections,
-            combine_text_sections: false,
             x86_formatter: self.x86_formatter,
             mips_abi: self.mips_abi,
             mips_instr_category: self.mips_instr_category,
@@ -284,6 +283,7 @@ impl DiffObjConfigV1 {
             arm_sl_usage: self.arm_sl_usage,
             arm_fp_usage: self.arm_fp_usage,
             arm_ip_usage: self.arm_ip_usage,
+            ..Default::default()
         }
     }
 }