make labCol order consistent with selectLabs

[TianyiShi2001]

Currently, when we have selectLab and labCol with multiple values, the order of the colors is not consistent with selectLab. An example (extended from the first example in the vignette):

library('DESeq2')
library(EnhancedVolcano)
library(airway)
data(airway)

dds <- DESeqDataSet(airway, design = ~ cell + dex)
dds <- DESeq(dds, betaPrior=FALSE)
res <- results(dds,
               contrast = c('dex','trt','untrt'))
res <- lfcShrink(dds,
                 contrast = c('dex','trt','untrt'), res=res, type = 'normal')

EnhancedVolcano(res,
                lab = rownames(res),
                x = 'log2FoldChange',
                y = 'pvalue', selectLab = c('ENSG00000125148', 'ENSG00000000419'),
                labCol = c('brown', 'gray'))

This PR resolves this issue by reordering the toptable subset according to the order of selectLab before plotting the labels.

Also I did some refactoring to minimize repetitions.